Objective: To determine whether children with calcium-deficiency rickets have a better response to treatment with vitamin D and calcium than with calcium alone. Design: Randomised controlled trial. Setting: Jos University Teaching Hospital, Jos, Nigeria. Population: Nigerian children with active rickets treated with calcium carbonate as limestone (approximately 938 mg elemental calcium twice daily) were, in addition, randomised to receive either oral vitamin D2 50 000 IU (Ca+D, n=44) or placebo (Ca, n=28) monthly for 24 weeks. Main outcome measure: Achievement of a 10-point radiographic severity score ≤1.5 and serum alkaline phosphatase ≤350 U/L. Results: The median (range) age of enrolled children was 46 (15–102) months, and baseline characteristics were similar in the two groups. Mean (±SD) 25-hydroxyvitamin D (25(OH)D) was 30.2±13.2 nmol/L at baseline, and 29 (43%) had values <30 nmol/L. Baseline alkaline phosphatase and radiographic scores were unrelated to vitamin D status. Of the 68 children (94% of original cohort) who completed 24 weeks of treatment, 29 (67%) in the Ca+D group and 11 (44%) in the Ca group achieved the primary outcome (p=0.06). Baseline 25(OH)D did not alter treatment group effects (p=0.99 for interaction). At the end of 24 weeks, 25(OH)D values were 55.4±17.0 nmol/L and 37.9±20.0 nmol/L in the Ca+D and Ca groups, respectively, (p<0.001). In the Ca+D and Ca groups, the final 25(OH)D concentration was greater in those who achieved the primary outcome (56.4±17.2 nmol/L) than in those who did not (37.7±18.5 nmol/L, p<0.001). Conclusions: In children with calcium-deficiency rickets, there is a trend for vitamin D to improve the response to treatment with calcium carbonate as limestone, independent of baseline 25(OH)D concentrations. Trial registration number: ClinicalTrials.gov NCT00949832.